The invention relates in general to large caliber gun tubes and in particular to the evacuation of propellant gas from such tubes.
Modem armored vehicles, such as tanks and self propelled howitzers, generally have a large caliber gun as the main armament. The breech of the gun may be located within a manned, closed-cab vehicle. After firing a round, residual propellant gases in the gun must be evacuated from the bore to prevent them from entering the cab of the vehicle. Propellant gases may contain noxious components such as carbon monoxide, nitric acid, and ammonia. If the propellant gases enter the vehicle compartment, they may pose a hazard to the crew. In addition, the fuel-rich propellant gases may spontaneously combust (known as “flareback”) when they contact the air in the cab. Various means of purging propellant gases from the bores of guns have been developed and used over the years.
A conventional approach to this problem has been the integration of a reservoir (alternatively called a bore evacuator or a fume extractor) located approximately mid-bore and tapped into the bore by a series of ports angled towards the muzzle. During the firing cycle, propellant gases enter the angled ports and charge the reservoir to approximately 100 psi. After firing the round, the pressure in the bore drops to atmospheric, the process reverses itself, and gases begin to discharge back into the bore through the angled ports. The action of this process is such that, when the breech is opened, air from the cab is entrained at the breech into the bore and the residual propellant gases are ejected out the muzzle. Because this process happens very rapidly (the entire cycle lasts approximately one second), the timing of the breech opening is crucial to performance.
Armored systems which use bore evacuators or fume extractors are recognizable by the characteristic reservoir located approximately mid-bore of the main armament. Such systems are advantageous because of their passive operation and can be very effective if properly maintained. However, there are some deficiencies in the conventional evacuators.
To be effective, such a system must be able to operate over a rather wide range of propellant charge pressures. In tank guns, the system must be capable of effectively evacuating the bore with standard high pressure rounds, as well as newer low pressure rounds. Artillery systems typically also use varying charge zones which can vary over an order of magnitude in pressure.
In addition, the trend towards lighter-weight guns necessitates longer recoil and hence, delayed breech opening. Delayed breech opening limits effectiveness of the conventional evacuator. The conventional bore evacuator must be significantly over-designed to properly function under a wide variety of conditions. Consequently, conventional evacuators may be complicated, heavy devices that may add undue weight to the weapon system, adversely affect the center of gravity of the vehicle or system, and require maintenance. The location of the bore evacuator (external to the vehicle) also makes them vulnerable to damage. A single puncture from small arms fire, shrapnel, or impact can render the bore evacuator non-functional. For effective operation, such systems may require cleaning and maintenance after each firing day.
A need exists for a bore evacuator that is more efficient over a range of propellant charge pressures, less vulnerable to damage, and requires less maintenance than a conventional bore evacuator.